Batteries, coupled with an electronic regulator, are a conventional energy storage device where energy from unregulated power supplies may be stored and regulated. In applications where the ratio of weight and/or volume to energy storage capacity, and absolute weight and volume, are critical, such as satellite systems, batteries are generally a disadvantage. This is especially so if the batteries will be required to undergo frequent deep discharging and re-charging. Deep discharging and re-charging severely shortens the life of batteries. One approach is to use more battery capacity so that the discharge will be less deep. However, this solution increases the system weight and volume. Batteries also suffer from the shortcoming that they are untestable as to their state of charge: the only true test of a battery's power is to apply a load across its terminals and thus waste the stored energy. This is an unacceptable situation in satellite systems.